“Cap in the swimming pool, cap in the sea... We don't have to constantly wear a swimming cap on our heads surely?!”
Ah there you are! The great critic of swimming caps, I thought I recognised you!
Rest assured, it is not mandatory. Just be aware that a swimming cap can be beneficial for your comfort when swimming but also and especially for your safety!
Beyond keeping your head warm and out of the water thanks to its buoyancy, wearing a brightly coloured swimming cap will prevent other people in the water (boats, surfers, etc.) from colliding with you and help those on the shore to spot you from far away in case of a problem.
And also... salt water is aggressive, think of your poor hair at least...